1. Field of Invention
This invention pertains to a composition of matter and a method of preparing it. More precisely, this invention involves the chemistry of organo-silicon compounds and a new compound of this class. Specifically, this invention reveals the compound 1,4-dimethyl-1,4-disilacyclohexadiene-1,4-di(alkali metal silanoate) and a method of making it. This compound is useful as a starting material in the manufacture of silicon containing polymers, which are particularly well suited for use in high temperature applications such as insulators, adhesives, potting agents, and composite materials.
2. Description of the Related Art
The silicone rubber industry is based on a chemical process called the direct process, wherein methyl chloride is reacted with elemental silicon to produce mixed methylchlorosilanes. The monosilicon methylchlorosilanes are then used to produce conventional silicone rubbers. The direct process produces a byproduct known as direct process residue which consists of methylchlorosilanes having multiple silicon atoms. These higher methylchlorosilanes can be processed chemically to monosilanes to increase the yield of the direct process, or they can be used as the starting material to make other organosilicon compounds.
The fraction of direct process residue that boils in the range 150.degree.-160.degree. C. is a source of methylchlorodisilanes. A method for isolating 1,2-dimethyltetrachlorodisilane from this fraction and then converting it to 1,2-dimethyltetramethoxydisilane has been reported by Watanabe et al in the Journal of Organometallic Chemistry, 128 (1977) 173-175. Watanabe's technique involves two steps. The disilane fraction is first chlorinated with dry hydrogen chloride in the presence of aluminum chloride to convert the unwanted trimethyltrichlorodisilane into dimethyltetrachlorodisilane. The dimethyltetrachlorodisilane is next purified by distillation and then treated with methylorthoformate to replace the chlorine atoms with methoxy groups. The result is 1,2-dimethyltetramethoxydisilane. The 1,2-dimethyltetramethoxydisilane resulting from Watanabe's technique can be converted into 1,4-dimethyl-1,4-dimethoxy-1,4-disilacyclohexadiene by the method disclosed in Atwell's U.S. Pat. No. 3,465,018. Atwell's method consists in part of reacting a substituted tetramethoxy disilane precursor with an acetylene at elevated temperature to produce a compound with the disilacyclohexadiene ring structure. Atwell's patent claims a broad class of disilacyclohexadienes with various functional groups attached to the silicon atoms in the ring. Hydrolyzable groups are specifically claimed and multiple examples are given of these substituent groups. Hydrolyzable functional groups on the silicon atoms of the ring lend themselves to further chemical reactions, most notably polymerizations.
Example 7 in Atwell's patent shows a polymer made by exposing a dihydroxy substituted disilacyclohexadiene ring compound to acid catalyzed condensation polymerization. This is a standard polymerization technique wherein water is eliminated from two hydroxyl groups, allowing an oxygen atom to bridge together two monomer molecules. The process continues until many monomers are connected. The resulting polymer, 1,4-dimethyl-2,3,5,6-tetraphenyl- 1,4-polydisilacyclohexadienol, is the only known disilacyclohexadiene ring polymer in the prior art. This polymer is reported to have a melting point ranging from 10.degree. C. to 320.degree. C. Such a range of melting points indicates that the polymer is not an elastomer and that it has a very broad molecular weight distribution. Moreover, complete melting at 320.degree. C. indicates that the thermal properties are substantially inferior to other well known polymers, including many commercial polysilanes and polysiloxanes.
Disilacyclohexadiene ring polymers must be commercially viable to be truly useful. The diphenylacetylene used in the synthesis of Atwell's ring polymer is expensive and relatively rare. The applicants therefore chose to use ordinary, inexpensive acetylene gas to synthesize the disilacyclohexadiene ring structure. The result is a ring without substituent groups at the 2,3,5,6 positions. The chemistry of this "naked ring" is different than that of a ring with bulky phenyl groups attached to it. For this reason the diol of the unsubstituted ring could not be isolated. Attempts to do so led to uncontrolled polymerization and useless gooey masses. A new approach to condensation polymerization would be needed to allow unsubstituted disilacyclohexadiene to polymerize in a controlled way.
While the diol of unsubstituted disilacyclohexadiene ring proved to be very difficult to isolate, isolating the dialkali metal salt of the diol proved to be straightforward. Moreover, the dialkali metal salt can be made without first making the diol. The dialkali metal salt was found to behave like a base in the presence of acids, forming the alkali metal-acid salt and ring diol. The ring diol thus formed immediately polymerized in a controlled manner. The use of a alkali metal salt as a polymer precursor is an unconventional technique that in this case solved an otherwise intractable problem. The process can be understood as acid-base condensation polymerization, and the dialkali metal salt of the diol of disilacyclohexadiene makes it possible.
This invention provides a starting material capable of forming disilacyclohexadiene ring polymers and elastomers with outstanding thermal stability, superior to any previously known polymer. The new compound, 1,4-dimethyl-1,4-disilacyclohexadiene-1,4-di(alkali metal silanoate), is derived from one of Atwell's compounds and is able to readily enter acid-base condensation polymerizations under acidic conditions. The new compound is made from inexpensive materials, providing the potential for commercial manufacture. Both homo and copolymers have been made and tested which utilize this starting material (See copending applications by the applicants Ser. No. 08/070,031, titled Oxodimethyldisilacyclohexadiene Homopolymers, filed May 28, 1993; and, Ser. No. 08/074,097 titled Oxodimethyldisilacyclohexadiene-Siloxane Co-polymers, filed Jun. 7, 1993).